1. Field of the Invention
The present invention relates to a phosphor and a light emitting device using the same. The light emitting device is used in signal lights, illumination apparatuses, displays, indicators and the like.
2. Description of the Related Art
Light emitting devices constituted from a combination of a light source and a phosphor which emits light when excited by light from the light source has been widely known hitherto. These light emitting devices include one that combines a light source emitting light in a region ranging from near-ultraviolet rays to visible light of shorter wavelengths and a phosphor which emits light when excited by the light from the light source. This type of light emitting device uses a phosphor having a composition of, for example, ZnS:Cu, (Sr, Ca)Ga2S4:Eu or SrAl2O4:Eu. However, such phosphor has low weatherability and low light emission efficiency. Thus there is a demand for phosphors which emit light over a region ranging from green to yellow light with high weatherability and high light emission efficiency.
In recent years silicate-based phosphors have been known which emit light over a region ranging from green to yellow light, such as (Ca0.99Eu0.01)2MgSi2O7 (refer to, for example, JP 2003-306674A). The silicate-based phosphors are excited with high efficiency by light of wavelengths ranging from 350 nm to 500 nm emitted by blue light emitting element, and emit yellow light.
There is a silicate-based phosphor having a composition of (Sr0.93Ba0.05Eu0.02)2SiO4 (refer to, for example, JP 2004-115633A). This silicate-based phosphor emits yellow light with peak emission wavelength of 570 nm when excited by monochromatic light in a wavelength ranging from 120 nm to 550 nm. There is also a silicate-based phosphor having the composition of (Sr0.37Ba0.6Ca0.01Eu0.02)2SiO4 (refer also to JP 2004-115633A). This silicate-based phosphor emits green light with peak emission wavelength of about 522 nm.
Furthermore, there is also known yttrium-aluminum oxide phosphor (hereinafter called YAG phosphor) which is excited by light in a region ranging from near-ultraviolet rays to visible light of shorter wavelengths and emits yellow or green light. The YAG phosphor emits light of high brightness and has high weatherability.
However, the phosphors emitting yellow or green light currently available do not have sufficiently high efficiency of emission for use in light emitting devices including the applications to display and illumination, and it is called for to further improve the brightness of light emission and the mass productivity. Particularly, there is known no phosphor which is capable of emitting green or yellow light with a high brightness comparable to that of the YAG phosphor. Thus there is such a problem that emitting light including much green component requires the phosphor to include a high proportion of green light emitting phosphor which has low efficiency of emission, thus resulting in lower relative brightness.